Volatile emissions from crude oil in stock oil tanks is regulated by the Environmental Protection Agency's New Source Performance Standards (NSPS, 40 CFR Part 60 Subpart OOOO dated Aug. 16, 2012). The NSPS applies to storage tanks used in oil or natural gas production with the purpose of reducing toxic air pollutants and Volatile Organic Compound (VOC) emissions. Concurrently, recent reports indicate that crude oil from new shale plays have become a transportation safety risk. The concern is that the high volatility, measured by the Reid Vapor Pressure (RVP), from the Bakken Shale formation in North Dakota and the Eagle Ford Shale formation in Texas had RVP readings over eight pounds per square inch (PSI), and that some wells were producing oil with RVP readings as high as 12 PSI. A series of recent volatile crude oil railcar accidents have resulted in fires and deaths. Volatility risk also increases when crude oil is produced in a cold climate, and then shipped to a warm climate, because crude oil volatility increases exponentially with temperature. Consequently, oil and transportation industries are seeking solutions to reduce crude oil volatility and storage tank emissions.
Crude oil from a wellhead separator contains a copious amount of emulsified water at a pressure of 30 to 70 pounds per square inch gauge. The crude oil is sent to a heater-treater to break the oil and water emulsion. The separated crude oil is subsequently delivered to a stock oil storage tank, operated at ambient pressure. The transfer of crude oil from a hot, pressurized heater-treater to the ambient storage tank causes a substantial amount of VOC to vaporize as fugitive emissions. The NSPS regulation requires recovery of the VOC if emissions exceed 6 tons per year. The fugitive emissions contain a substantial amount of natural gas liquid (NGL) and natural gasoline. A Vapor Recovery Tower (VRT) upstream of the storage tanks may be used to separate the VOC from the crude oil. The VOC may be either burned or recovered in a vapor recovery unit (VRU). Vapor recovery units simply collect hydrocarbons from the vapor recovery tower, then compress the gas for transfer to a natural gas pipeline. However, about one-third of the wells in North Dakota are not connected to a pipeline. In such cases, the crude oil is transferred from the storage tanks to a transport tank (e.g. railcar tanks, tanker trucks, etc). For the wells that are connected to a pipeline, valuable hydrocarbons are sold at a discount when blended with natural gas.
A conventional oilfield operation is depicted in FIG. 1. Oil 1 from one or more local wells is collected into a gathering system (i.e. a matrix of rock crude oil collection pipes) and fed into separation vessel 2 where gas 3 flows from the top, water 5 flows from the bottom, and oil 4 from the side flows into heater-treater 6. Heater-treater 6 breaks the oil water emulsion and further separates oil and water. Vapor 7 from heater-treater 6 flows to flare 13. Water 9 flows from heater-treater 6 to water storage tank 10. Oil 8 is decanted from heater-treater 6, and then flows into oil storage tank 11. Oil storage tank 11 vents volatile organic compounds 12.
To conform to the new NSPS regulation, producers are inserting a Vapor Recovery Tower (VRT) 15 upstream of the crude oil storage tank 17 as depicted in FIG. 2. Oil 14 from the heater treater flows into VRT 15 where gas and oil are separated. Oil 16 from VRT 15 flows into crude oil storage tank 17. Gas from VRT 15 flows to a flare or combustion device 20. Alternatively, gas from VRT 15 may also be compressed into a pipeline. The crude oil tank vent 18 from crude oil tank 17 flows into VRT 15.